The present invention relates to the powering of automotive engine-cooling fans. Such fans move air through a radiator through which engine coolant flows. In some cases these fans also cool other heat exchangers such as a charge-air cooler, an oil cooler, or a refrigerant condenser.
The prior art consists of several methods of powering such a fan.
The method used in most light-truck applications is to drive the fan via an accessory drive belt powered by a crankshaft pulley. Typically the fan is attached to a viscous clutch, the engagement of which will vary according to the temperature measured by a bimetallic coil located on the face of the clutch.
The cooling capability of an engine-driven fan is directly proportional to engine speed. It therefore is a good choice for applications where the critical cooling requirement is encountered at high engine speeds (e.g., 3,000 rpm), such as when towing a heavy trailer up a grade. However, in the case of an air-conditioned vehicle, an engine-driven fan often delivers marginal air flow for condenser cooling at idle conditions.
In a typical application, the fan shaft is rigidly attached to the engine and the shroud is attached to the radiator, so that in order to allow for the movement of the engine on its mounts, a large clearance gap is required between the two. The large clearance gap limits the efficiency of this type of fan, but since the fan uses the engine's mechanical power directly, the net efficiency is competitive with other fan powering systems as long as the viscous clutch is fully engaged. When the clutch is not fully engaged, the efficiency is reduced by a factor equal to the engagement ratio of the clutch. Since the fan is often sized for the trailer-towing condition, the required fan speed is generally significantly less than the speed with clutch engaged, so the efficiency averaged over the life of the vehicle is poor.
Clutch durability is also a problem with engine-driven fans. To limit the fan power at very high engine speeds (e.g., 5,000 rpm), the engagement ratio of the viscous clutch is designed to decrease at these speeds. As a result, a large amount of power is dissipated in the clutch, which sometimes leads to clutch failure. When the viscous clutch operates properly, the engine power necessary to power the fans at very high engine speeds is still high due to the power dissipated by the clutch. This reduces the maximum engine power available for powering the vehicle.
Engine-driven fans have very demanding structural requirements. If the clutch locks up, and the engine runs at its maximum speed, the fan speed will be significantly higher than the design speed, and the stresses in the fan will increase by the square of that speed ratio. As a result of this high-speed structural requirement, the fans used on these systems are generally radial-bladed, free-tipped designs. The efficiency and noise advantages of skewed and banded fans cannot be realized.
Other disadvantages of an engine-driven fan system include the fact that it requires that the engine be mounted longitudinally in the vehicle, and limits the positioning of the fan and heat exchangers. In particular, a dual-fan arrangement is difficult to engineer with this system.
In most passenger cars the fan is driven with an electric motor powered from the vehicle electrical system. This system provides excellent idle cooling performance, but its capacity under trailer-tow conditions is limited by available electric power. There are many advantages to this system. An electric fan assembly can have small fan-to-shroud clearances, and can use banded fans. The high efficiency of such a fan can compensate for the inefficiencies of the alternator and motor, yielding a net efficiency comparable to that of an engine-driven fan at full clutch engagement. Skewed blades can be used to reduce noise.
Speed control of electric fans can be more sophisticated, and more efficient, than that of a typical viscous clutch. If the motor is electronically commutated, or a speed controller is included in the circuit, the fan can be run at a speed appropriate to the cooling needs of the vehicle, as determined by the on-board computer. An example of such a speed control system is described in U.S. Pat. No. 4,425,766. The disadvantage of these speed-control methods is that electronic devices capable of handling the high currents involved are quite expensive.
Another advantage to electric cooling fans is that the engine can be placed transversely in the vehicle, an arrangement that often optimizes space utilization. Dual-fan systems can minimize the height of the hoodline in order to achieve favorable vehicle aerodynamics.
Some fan systems combine an engine-driven fan with an electric fan in order to achieve good cooling at both idle and high engine speeds. The electric fan is sometimes arranged as a "pusher" fan upstream of the condenser, or as a "puller" to the side of the engine-driven fan. A hybrid fan system places an electric "puller" fan upstream of an engine-driven fan, in a counter-rotating arrangement. Although the electric fan increases the available air flow at idle, these systems have many of the disadvantages of engine-driven fans enumerated above.
A relatively recent development is the use of hydraulic motors to drive automotive engine-cooling fans. The motor is powered by either the power-steering unit or a dedicated pump. In order to avoid the high cost of a variable-speed pump, a bypass valve is employed for speed control. When the desired fan speed is less than the maximum speed available, some fraction of the hydraulic fluid is recirculated through the bypass valve. This method of speed control is quite inefficient, and results in heating of the hydraulic fluid. Often an additional heat exchanger must be added to the system in order to cool this fluid. This, in turn, increases the fan power necessary to cool the vehicle. A hydraulic system can deliver good cooling at both idle and high-speed conditions, but the overall efficiency of the system is not good.
To summarize, engine-driven fans offer good cooling at high engine speeds, but marginal cooling at engine idle and poor efficiency when cooling demands are moderate. Conventional electric systems cool well at idle, offer reasonable efficiency at all conditions, but don't offer the necessary capacity for heavy trailer-towing applications. Hydraulic systems offer good cooling at idle as well at trailer-tow conditions, but are inefficient at moderate fan speeds.
The present invention also relates to the operation of automotive coolant pumps. Conventionally, a coolant pump is driven directly by the accessory drive belt, with no viscous clutch. Since it runs at a speed proportional to engine speed, the power absorbed at high engine speeds can be considerably greater than that required to cool the engine. To avoid this power loss, electric coolant pumps have been proposed. Such a pump could be run at the speed required for proper engine cooling and heater operation, independent of engine speed.